HURLER SYNDROME - A PATIENT WITH ABNORMALLY HIGH-LEVELS OF ALPHA-L-IDURONIDASE PROTEIN

Mucopolysaccharidosis type I (MPS I: McKusick 25280) is a clinically heterogenous lysosomal storage disorder which is caused by a variable deficiency in α-l-iduronidase activity (α-l-iduronide iduronohydrolase, EC 3.2.1.76). Cultured fibroblasts from an MPS I patient (cell line 2827) with a severe clinical phenotype (Hurler syndrome) have been characterized using immunochemical and biochemical techniques. Using a specific immunoquantification assay, we have demonstrated that cell line 2827 had an α-l-iduronidase protein content (189 ng/mg of extracted cell protein) at least six times greater than the mean level found in normal control fibroblasts (30 ng/mg of extracted cell protein). This was the only MPS I cell line, from a group of 23 MPS I patients, that contained greater than 7% of the mean level of α-l-iduronidase protein detected in normal controls. Cell line 2827 had very low α-l-iduronidase activity toward the fluorogenic substrate 4-methylumbelliferyl-α-l-iduronide, and a radiolabeled disaccharide substrate derived from heparin. Maturation studies of α-l-iduronidase in cell line 2827 showed apparently normal levels of α-l-iduronidase synthesis with delayed processing to the mature form. Subcellular fractionation experiments demonstrated α-l-iduronidase protein in lysosomal-enriched fractions isolated from cell line 2827, suggesting a normal cell distribution and supporting the proposed delayed processing. It is proposed that the MPS I patient described has an α-l-iduronidase gene mutation which affects both the active site and post-translational processing of the enzyme. This mutation must be structurally conservative because it does not result in instability either during maturation or in the lysosome. © 1992.